By reason of their high heat capacity, fluidized beds have long been used for conducting all types of reactions bringing into play large amounts of heat energy, such as chemical reactions, gasification of hydrocarbon materials, production of cement or combustion of fuel material or of waste for the production of energy.
In certain cases, a fluidized bed of the circulating type is used, i.e., containing solid particles capable of being entrained with the gases emerging from the reaction chamber, then recovered in a separating device and recycled into the fluidized bed.
Circulating beds are used, for example, in gasification reactors or combustion chambers in which the burnt particles entrained with the smoke are recycled into the fluidized bed until combustion is completed.
The fluidized bed can also be constituted from inert particles. This is the case, for example, in French Pat. No. 2,209,074, which relates to a device for incinerating waste, in which the fluidized bed contains noncombustible particles which are entrained with the smoke and then, after separation, passed into a heat exchanger where they yield up their heat before being sent back into the combustion chamber, there to be reheated. In this method, the circulation of the particles only serves for the transfer of calories between the combustion chamber and the heat exchanger.
In addition, certain reactions must occur at a temperature which must remain relatively constant or indeed be maintained below a certain limit, for example to avoid the fusion of ash which can interfere with the operation of the fluidized bed, or indeed to carry out desulfuration.
To carry out temperature regulation of the fluidized bed, the latter is generally cooled either by injecting therein a moderator fluid such as air, water or steam, or by immersing therein tubes cooled by the circulation of a heat transfer fluid which can, for example, produce steam or hot water.
The injection of a moderator fluid into the bed, which is used notably in gasification reactors, has the drawback of reducing the quality of the gas produced. The cooling of the fluidized bed by immersed tubes avoids this drawback but limits the flexibility of the installations. In fact, the exchange coefficient is relatively constant and one is led therefore, in certain applications, to partition the fluidized bed into compartments and to adjust the number of the compartments according to the power required, this obviously complicates the installation.
It is an object of the invention to provide a method and a device which do not have these drawbacks while enabling the continuous adjustment of the temperature of the fluidized bed to a certain level.
For this purpose, the fluidized bed operates as a circulating bed for at least a part of the solid particles that it contains, and it is associated with a heat exchanger connected in parallel to the recycling circuit of the particles.